1. Field of the Invention
The invention relates to light emitting devices and more particularly to light emitting devices formed from nitride semiconductors. Hereinafter, “a light emitting device” refers to “a light emitting element” or “a mounted light emitting element construction equipped with a light emitting element”, unless otherwise mentioned.
2. Description of the Background Art
White light emitting diodes (LED) have been widely utilized for illumination for the display devices of portable information terminals, etc., including portable phones. In the case of using LEDs as the light source of the display device of a portable information terminal, it is required to enhance the light emission performance. Therefore, there has been suggested a configuration for preventing light emitting unevenness in fabricating a side-view type LED equipped with a GaN-type light emitting device which has been assembled using a sapphire substrate and n-down (p-top) mounted (see Japanese Laid-Open Patent Publication No. 2000-223751). According to this suggestion, there is disclosed that two electrodes at opposite corners of the rectangular shape, in a plane view of the light emitting device from the light extracting side (top side), are placed such that they are lied along the thickwise direction of the side-view type LED, namely they stand at the corners.
Further, there is a possibility that LEDs will be utilized for illumination for large spaces or large areas. There is a need to increase the light output efficiency of LEDs either for large area illumination or for application to portable information terminals.
FIG. 51 illustrates the construction of a GaN-type LED which has been currently suggested (see Japanese Laid-Open Patent Publication No. 2003-8083). In this GaN-type LED, an n-type GaN layer 102 is provided on a sapphire substrate 101, and a quantum well construction 103 is formed between n-type GaN layer 102 and a p-type GaN layer 104. Light emission occurs at this quantum well construction 103. On p-type GaN layer 104, a p-electrode 105 is formed such that it is in ohmic-contact therewith. Further, on n-type GaN layer 102, an n-electrode 106 is formed such that it is in ohmic-contact therewith.
These p-electrode 105 and n-electrode 106 are connected to a mounting member 109 through solder balls 107, 108. Mounting member (submount member) 109 is formed from a Si substrate and is provided with a circuit for protecting the light emitting device from surge voltages from the outside. Namely, considering that main factors of circuit failures for III semiconductors such as Ga, Al and In are surge voltages such as transient voltages or electrostatic discharge, an electric power shunting circuit for protecting the light emitting device is formed from Zener diodes, in order to prevent the light emitting device from being subjected to large forward currents and reverse currents. The protection from surge voltages will be described in detail later.
The aforementioned GaN-type LED is characterized in that (a1) p-type GaN layer 104 is down-mounted and (a2) n-electrode layer 106 is formed on n-type GaN layer 102. This GaN-type LED has a significantly complicated construction as can be seen in FIG. 51. The reason that (a2) the n-electrode layer is formed on n-type GaN layer 102, which makes the construction complicated, is that sapphire substrate 101 is an insulator and the n-type electrode can not be provided on the sapphire substrate.
For light emitting devices using GaAs-type, GaP-type, and GaN-type compound semiconductors, as well as the aforementioned light emitting device using a sapphire substrate, there has often been suggested that a circuit for protecting the light emitting device from transient voltages and electrostatic discharge is provided in conjunction with the light emitting device (see Japanese Laid-Open Patent Publication Nos. 2000-286457, 11-54801, and 11-220176). Particularly, GaN-type compound semiconductors have small reverse withstand voltages such as about 50 V, and also have forward withstand voltages of only about 150 V. Therefore, it is considered important to provide aforementioned electric power shunting circuit for protection. Namely, the aforementioned GaN-type device, etc., is formed on a submount Si substrate and on the Si substrate a protection circuit including Zener diodes is provided. A plurality of suggestions of protection circuits as described above is proof of that main factors of circuit failures for III semiconductors such as Ga, Al and In are surge voltages such as transient voltages or electrostatic discharge.
Besides the aforementioned light emitting device provided with a protection circuit, there has been known an example where a GaN-type light emitting device is formed on a conductive SiC substrate. Namely, there has been widely known LEDs configured to have a laminated-layer construction as follows to emit light from the p-type GaN layer: (an n-electrode on the back surface of a SiC substrate/SiC substrate/n-type GaN layer/quantum well laminated-layer construction (light emitting layer)/p-type GaN layer/p-electrode).
On the other hand, the aforementioned GaN-type LED using a sapphire substrate illustrated in FIG. 51 has a complicated construction, which unavoidably increases the fabrication cost. Since it is necessary that LEDs are inexpensive in order to develop demand in various illumination applications, the aforementioned construction is not preferable. Further, since p-electrode 105 and n-electrode 106 are placed on the down-mounting surface side, the areas of the electrodes, particularly the area of the p-electrode, is restricted. In order to flow large currents to provide high outputs, it is desirable that the p-electrode has a large area. However, the construction illustrated in FIG. 51 restricts the areas, thus restricting the light output. Further, in view of discharging heat generated in association with currents, it is not preferable that the two electrode layers are placed on one surface.
Further, there is a large resistance to currents flowing in the direction parallel to n-type GaN layer 102, which may cause heat generation and, therefore, increases in the power consumption. Particularly, in the case where the thickness of the n-type GaN layer is reduced in order to shorten the film formation processes, the yield of exposure of the n-type GaN film is degraded, in addition to the aforementioned heat generation and power consumption increases.
Further, it can be said for light emitting devices in general that the heat radiating area is restricted and also the heat resistance (the temperature rise due to unit energy introduced per unit area) is large, and therefore the injected current per single light emitting device can not be made large. Particularly, in the case of using a sapphire substrate, the area of the p-electrode is restricted as previously described and it is usually to perform heat designing with little margin.
Further, since the aforementioned GaN-type LED using a sapphire substrate restricts the heat radiating area, it unavoidably becomes necessary to utilize a configuration including intricate comb-shaped p-electrode and n-electrode for increasing the contact area therebetween. It is not easy to manufacture these comb-shaped electrodes, thereby certainly increasing the manufacture cost.
As previously described, the design of heat conditions is basically important for light emitting devices. When an attempt is made to generate high outputs, the aforementioned heat conditions introduce restrictions thereto, and therefore it is unavoidably required to use complicated electrodes in order to alleviate them as much as possible.
Further, there is a problem as follows. When a GaN-type light emitting device formed on a sapphire substrate is down-mounted and the back surface of the sapphire substrate is used as the light emitting surface, light with a predetermined incident angle or greater will be subjected to total reflection at the boundary between the GaN layer and the sapphire substrate after propagating through the GaN layer and is not emitted to outside, since GaN has a refractive index of about 2.4 and the sapphire substrate has a refractive index of about 1.8. Namely, light with an incident angle θ equal to or higher than sin−1(1.8/2.4)≈4.2° will be confined within the GaN layer and will not be emitted to the outside. This reduces the light emitting efficiency at the main surface of the sapphire substrate. This problem of the light emitting efficiency is important. However, there are still other problems. The aforementioned light subjected to total reflection will propagate through the GaN layer and then is emitted from the side portions of the GaN layer. The energy density of the light emitted from the side portions will be high, since the ratio of the amount of the aforementioned totally-reflected light is considerable and also the GaN layer has a small thickness. This will damage resin which is placed at the side portions of the GaN layer and thus irradiated with the light. This will induce a problem of shortening the life of the light emitting device.
Further, with a GaN-type LED having a construction of (an n-electrode on the back surface of a SiC substrate/SiC substrate/n-type GaN layer/quantum well laminated-layer construction (light emitting layer)/p-type GaN layer/p-electrode) which emits light from the p-layer side, light can not be efficiently emitted to the outside because of the high light absorption at the p-electrode. If an attempt is made to reduce the coverage ratio of the p-electrode, namely increase the opening ratio, in order to increase the amount of emitted light, currents can not be flowed through the entire p-type GaN layer since the p-type GaN layer has a high electric resistance. Therefore, light emission can not be activated through the entire quantum well construction, thereby decreasing the light emission output. Furthermore, the electric resistance will be increased, which will induce problems of heat generation and power supply capacity. Also, if an attempt is made to increase the thickness of the p-type GaN layer in order to uniformly flow currents through the entire p-type GaN layer, this will restrict the output since this p-type GaN layer will absorb a large amount of light.